In a significant number of medical treatments administered by medical practitioners treating various functions and disorders of the human eye, a need arises to deal with the shape of the outer layer or cornea of the eye. As is known, the cornea is a thin somewhat flexible generally transparent tissue forming the outer coat of the eye which has a nearly spherical curvature. As a result, the human cornea may be regarded approximately as a spherical specular object having negative reflecting power. The latter term refers to the fact that when light is reflected from the cornea its divergence increases. Of particular interest to the present invention, is the need which arises to measure the radius of curvature and topography or shape of the anterior or front service of the cornea. It is often necessary to undertake cornea measurements both statically and dynamically in different diagnoses and treatments. The measurement of cornea surface curvature, shape or topography is referred to as keratometry. To meet the needs of keratometry, various instruments referred to as keratometers have been developed. In most cases, such instruments utilize classical imaging techniques.
Conventional keratometers are limited to measurement only of cornea surface curvature. They cannot map the surface contours of the cornea with high resolution. In addition, they have a slow response time and are unable to measure dynamic changes in surface shape. Finally, they tend to obscure or interfere with the field of view of the examining practitioners or operating surgeons which in turn limits its degree of use.
There arises, therefore, a need in the art for an improved system of keratometry that accurately measures cornea surface topography in real time without interfering significantly with the view of treating surgeons and other practitioners.
The present invention applies moire deflectometry techniques to keratometry. The basic principles of moire deflectometry are illustrated and discussed in detail in a number of publications including an article entitled MOIRE DEFLECTOMETRY: A RAY DEFLECTING APPROACH TO OPTICAL TESTING by Okafri and Iglatt published in OPTICAL ENGINEERING, 24(6), 944-960(Nov./Dec. 1985). As is set forth therein, moire deflectometry utilizes a collimated light beam which is directed toward a parallel pair of optical gratings having alternating equally spaced opaque and clear stripes. The stripes of the two gratings are oriented at a small angular displacement with respect to each other. The interaction of the grating elements and the collimated light beam produce a fringe light pattern on a receiving device such as a screen. As an alternative to a conventional screen, a TV camera utilizing a charge coupled device detector array may be utilized to produce electronic formatted video images.
The basic moire deflectometer described above produces the desired measurement of a lens shape when the light beam incident upon the gratings has been reflected from the to-be-measured object. When so used, the fringe pattern referred to as a moire deflectogram produces a fringe pattern indicative of the shape of the reflecting object. To properly map the two-dimensional shape of an object, two measurements are required in which the reflected light from the object is passed through grating pairs which are rotated ninety degrees between measurements. In most of the moire deflectometers presently utilized for optical measurements, the collimated light beam is usually provided by a helium neon laser having a wavelength of approximately six hundred thirty three nanometers.
While the foregoing described conventional moire deflectometers have achieved some level of success for use in optical testing, their use for keratometry would be subject to several problems and limitations. One of the most significant limitations arises from the dependence of the system's accuracy upon the careful placement or measurement of the distance between the deflectometer and the cornea surface. That is to say, the measurement made is extremely sensitive to this distance and careful calibration must be utilized. In addition, great difficulty is experienced in maintaining a static position of the human eye being examined which further exacerbates the problem.